Hydraulic sorting of heavy-mineral grains by swash on a medium-sand beach

Hydraulic sorting of detrital mineral grains in the swash zone was investig ated using data on sedimentology and how dynamics obtained from Fishermans Beach, on the east coast of Australia, The beach is characteristically refl ective, displaying a steep beach face composed of medium sand and virtually no surf zone. Samples were taken from two beds enriched in heavy minerals, and from the adjacent beds above and below The mineralogy of the samples w as determined, and measurements of grain size and settling velocity were ob tained. The heavy-mineral grains had both a smaller intermediate diameter a nd a smaller settling velocity than the light-mineral grains; the differenc e between the two mineral types was typically in the range 0.25-0.50 phi an d 2.0-2.5 cm s(-1), respectively, On the basis of these sedimentological me asurements alone, any one of three mineral sorting mechanisms could provide a feasible explanation for the development of the heavy-mineral-enriched b eds: suspension sorting, entrainment sorting, or sheer sorting, A numerical model for smash, based on the nonlinear shallow-water wave theory, is used to provide a quantitative description of flow dynamics in the swash zone a t the time the heavy-mineral-enriched beds were formed. For the probable wa ve height responsible for producing the enriched beds, modeled maximum how velocity and bed shear stress at the mid-swash position reached 3.5 m s(-1) and 79.51 dynes cm(-2), respectively. Given the shallow water depths and t he lack of sorting in the horizontal direction of flow, suspension sorting does not seem to be of overriding importance at Fishermans Beach. The large bed shear stresses seem to also preclude entrainment sorting, because for most of the time all mineral types are predicted to be in motion. This cont rasts with previous studies on finer-grained beaches, where entrainment sor ting was favored, and suggests that beach type may play a role in determini ng the sorting mechanism operating at a particular location, Shear sorting was found to be feasible within the modeled flow constraints at Fishermans Beach, but it does not provide an entirely satisfactory explanation for the observed mineral sorting patterns, We contend that the inability to unequi vocally identify the true sorting mechanism is more a reflection of our poo r understanding of the operation of these sorting mechanisms, particularly within the swash zone, rather than an indication that any of these mechanis ms are physically inoperative in the swash zone.